Dog Flies (External Parasites)

Flies and Dogs - Definition

Flies are bothersome external parasites that cause irritation and also transfer significant disease to our pets. Their life cycle involves a complete metamorphosis, from egg to larva to pupa to adult. Fly larvae are called “maggots.” Agricultural animals, including horses, sheep, cattle, goats and pigs, tend to be more severely affected by flies and maggots than companion animals. However, flies do cause a number of medical problems for domestic dogs.

How Flies Affect Dogs and Why We Worry About Them

Let’s face it, flies are annoying. They cause irritation and itchiness simply by landing on someone’s skin. Some flies bite and then feed on their host’s blood or tissues, which can be painful and cause the dog to lick, chew, bite, rub or scratch at its skin in an attempt to relieve the discomfort. Flies that remove a great deal of blood (this usually happens when there are many flies feeding on a single animal) can cause a dog to become anemic. Anemia is an abnormal reduction in the number of circulating red blood cells. Red blood cells are necessary to deliver oxygen and other key nutrients throughout the body and to remove waste products from all tissues.

Flies often mechanically damage a dog’s skin in the areas around their bites. This predisposes those areas to secondary bacterial infections. Flies are considered by most people to be filthy insects. Adult flies pose a potential health risk not only to dogs, but also to people. They act as airborne couriers of bacteria and other infectious microorganisms, especially those found in feces, on their dirty little feet. Flies can deliver their unhealthy loads virtually anywhere and to anyone, causing a wide range of potential adverse consequences to the unlucky recipients.

Almost all flies are a nuisance to some extent. However, some flies affect dogs more seriously than others. Certain flies land and lay their eggs on the surface of a dog’s skin and fur. When these eggs are deposited near wounds or other raw areas, the resulting larvae (maggots) can easily penetrate the dog’s skin, causing severe irritation and creating an environment that is ripe for bacterial infection. Infestation of living animals with the larvae of flies is called “myiasis.” Myiasis, also known as “maggot infestation,” is most prevalent seasonally during warm-weather months, especially in hot, moist climates. Myiasis is considered to be a disease of neglect. Old dogs, dirty dogs, wounded dogs and weak dogs are especially vulnerable to maggot infestation. Many ordinary flies, including house flies, stable flies, horn flies and black flies, can cause myiasis in debilitated animals. Most flies and their larvae are fairly species-specific, which means that they tend to live on certain animals.

Sometimes, owners know that fly larvae have infected their dogs, but often they do not. Dogs infested with maggots may be taken to a veterinarian only because they develop a nasty, putrid smell. Occasionally, maggots are discovered incidentally during a routine physical examination, or when a dog is seen by a veterinarian for some unrelated reason. Whether or not the insects cause actual damage, disease or distress, most owners find it extremely unpleasant to see their dogs covered with flies. They usually are even more disturbed by seeing maggots feeding on their pets.

Specific Flies That Bother Dogs

One of the fly species that adversely affects dogs is the bluebottle fly or blowfly, known as Calliphora. Blowflies are more common in agricultural animals than in pets. However, they can infect companion dogs, and when they do their presence is particularly alarming. Adult blowflies have a blue to greenish metallic sheen to their bodies. Females prefer to lay their eggs on badly soiled, matted fur and on moist, devitalized, exposed areas of skin. Blowfly eggs typically hatch within three days of being laid. The maggots grow over the next few weeks, feeding on the dog’s skin and tissues. They eventually produce a special salivary enzyme that is capable of liquefying intact skin. As the maggots feed, this destructive salivary enzyme actually digests the dog’s skin, creating full-thickness, punched-out skin lesions (sores) within a matter of hours. The maggots move into these cavities. They continue feeding on exposed skin and enlarging the raw openings, creating a perfect environment for severe secondary bacterial skin infections. Blowfly larvae also secrete toxic substances into these wounds. If a dog becomes heavily infested with maggots, it can go into shock. This is a life-threatening medical emergency requiring immediate veterinary intervention.

Dogs with cutaneous (skin) myiasis need to have the maggots removed from their skin wounds. Mechanical removal is the most efficient way to accomplish this, and is probably best done by a veterinarian. Affected areas will be clipped closely to remove hair mats and then rinsed gently to remove surface dirt. The maggots can be pulled out with blunt-nosed tweezers or fine-nosed forceps, using gentle, constant traction. Care will be taken not to cut or crush the larvae. They will be killed and disposed of, probably in a tightly-sealed container placed into an outdoor garbage receptacle. After the larvae are removed, infected skin areas will be washed with Betadine or another disinfectant solution. Most authorities recommend bathing the dog thoroughly with a shampoo that contains pyrethrin or pyrethroid medication but is not alcohol-based. If the dog has secondary bacterial skin infections, the veterinarian may recommend a course of broad-spectrum oral and/or topical antibiotics. Maggots can also be killed by administering certain drugs, such as ivermectin, either orally or under the skin (subcutaneously) by injection. Treatment with ivermectin is not recommended for herding breeds or dogs that have tested positive for heartworm infection, as they can be extremely sensitive to this medication and can have severe, potentially fatal reactions.

Another fly that bothers companion dogs is Cuterebra, commonly called the rodent bot fly. This is a large fly with wide seasonal distribution across the United States during the warmer months. Female Cuterebra adults lay their eggs on the ground, close to burrows of rabbits and rodents. Dogs become infected by direct contact with bot fly eggs or their post-hatching parasitic larvae, which are called “grubs” rather than maggots. Although their preferred hosts are wild rodents, newly-hatched Cuterebra grubs can and do penetrate the unbroken skin of dogs. Importantly, they will also burrow into the skin of people. They settle just under the skin, forming cyst-like cavities in which they feed and grow. Maturing Cuterebra look like skin-covered lumps that have small slits in the center through which the larvae breathe. Multiple grubs can be located in the same pocket or in close proximity to one another. When this happens, they look like a large, lumpy tumor that resembles a small, smooth cauliflower located right under the skin.

Cuterebra grubs frequently settle under a dog’s belly, along the flanks and under the jaw. As they mature, their heads or breathing tubes may be seen poking out of the breathing slits in the dog’s skin. Roughly one month after they first burrow into the skin of their host, when they reach about one inch in length, the grubs emerge from their hiding places and drop to the ground. This entire process can be horrifying for owners of affected dogs, especially if they are unfamiliar with the Cuterebra life cycle.

If Cuterebra grubs are suspected or identified under a dog’s skin, it probably is best for the owner to take the dog to its veterinarian. The doctor will clip around the lumps to expose the larval breathing holes. Sometimes, the grubs can be grabbed with fine-tipped forceps and gently pulled out through those slits with constant traction. The veterinarian will be careful not to crush the larvae or rip them apart, if at all possible. It is much more difficult to extract grubs once they have torn. More importantly, if they rupture, Cuterebra grubs release toxins that enter the dog’s bloodstream and can cause severe allergic reactions, including anaphylactic shock. This can be life-threatening. If a grub cannot be pulled out or if it becomes damaged, the veterinarian can make a small surgical incision so that the parasite can be removed cleanly. Local anesthesia is usually necessary for this procedure. The wounds left by grubs are prone to heal slowly and to develop secondary bacterial infections. If this occurs, the attending doctor probably will prescribe a course of oral antibiotics.

Special Notes

Fortunately, there are many things that dog owners can do to keep flies under control. One of the most important and effective ways to manage flies is to keep a dog’s living environment clean and free from accumulating fecal matter. Outdoor garbage receptacles should be kept covered. If other animals – especially agricultural animals such as horses, cattle or pigs, but also rabbits, chickens or outdoor cats – live nearby, fly control at their living facilities will also be important. Fly traps are available at local hardware stores, feed stores and elsewhere and usually are quite effective in helping to reduce fly numbers. They come in the form of plastic containers, glass jars, sticky tape and cardboard window traps, among others. Generally, fly traps are relatively inexpensive. Topical preparations used to control fleas and ticks may also be effective in repelling flies; these should be discussed with the dog’s veterinarian in advance of being used for this purpose.

Regular grooming will give owners an opportunity to inspect their dogs’ skin and coat. If open wounds or suspicious lumps are present, whether or not maggots or grubs are seen, a trip to the veterinarian is probably worthwhile. While prevention is the best cure, quick identification and treatment of fly-related medical conditions is the next best thing.